Combustion system and method for power generation

ABSTRACT

A power generating combustion system and method are provided with a first stage combustion system that may include a wet oxidation reactor or a direct contact boiler and a second stage combustor including a stoichiometric burner to produce the substantially complete combustion of a wide range of fuels and mixtures of fuels to produce combustion products usable in generating power. The system design temperatures and pressures allow the substantially complete combustion of garbage, municipal and industrial waste and low quality fuels to generate power.

TECHNICAL FIELD

The present invention relates generally to combustion systems andspecifically to a combustion system and method, particularly oneincorporating a wet oxidation reactor or direct contact boiler, capableof combusting a wide range of types of fuels and fuel mixtures togenerate power without producing toxic or polluting power plant efflux.

BACKGROUND OF THE INVENTION

The complete combustion of fuel, especially fuel from such diverse andlow quality sources as low grade fuels and municipal and industrialwaste, to generate usable power without concomitantly creatingenvironmental problems has remained an elusive goal yet to be fullyachieved. Low grade fuels tend to contain high amounts of sulfur andproduce the sulfur oxides implicated in acid rain when they are burnedin power plants. Municipal and industrial waste has traditionally beendisposed of in landfills. However, not only is there an acute shortageof landfill areas, but burying this waste has been demonstrated tocontaminate the surrounding soil and adjacent ground water with toxicsubstances. Some industrial waste, such as that containing low levelradiation, for example, is particularly hazardous and difficult todispose of without creating environmental or health problems.

Many industrial plants and communities have resorted to burning theirwastes. Although recycling programs have produced some reductions in thequantity of municipal and industrial waste to be disposed of, a vastquantity of such waste still remains to be disposed of by burning orotherwise. The burning or incineration of waste presents many seriousproblems. Existing incineration plants inevitably discharge pollutantsin excess of clean air standards, despite efforts to minimize thedischarge of these pollutants by the use of scrubbers, filters andelectrostatic precipitators. Moreover, the incineration processgenerates toxic ash, which must be disposed of.

The waste management industry has employed wet oxidation reactors forthe combustion of organic wastes, including those generated by garbage,pulp and paper operations and milk processing, to produce mainly carbondioxide and water. A typical process introduces a waste stream and airto a reactor at high temperature and pressure, which results in theoxidation of the organic matter in the waste stream in an exothermicreaction that may generate usable heat if the reaction is controlled.Low grade solid fuels may also be burned in a wet oxidation reactor. Thecareful control and monitoring required for wet oxidation reactors hasbeen one of their major disadvantages. In addition, the available wetoxidation reactors are thermally inefficient and do not producesubstantially complete combustion of the organic wastes or fuel. U.S.Pat. Nos. 4,053,404 to Van Kirk; 4,384,959 to Bauer et al.; 4,670,162 toRobey; and 4,721,575 to Binning et al. are illustrative of prior art wetoxidation reactors and wet oxidation reaction processes.

The prior art has proposed combustion systems for municipal waste, coal,and the like which permit the recovery of valuable resources and produceenergy useful for power generation or in a heat exchanger. U.S. Pat.Nos. 3,986,955 to Plique and 4,829,911 to Nielson are exemplary of suchsystems. These systems, however, are not thermally efficient because thewet oxidation process provides the only combustion system and is notproperly controlled to effect complete combustion. As a result, themaximum operating temperatures are not likely to be high enough to allowsubstantially complete combustion of the organic reactants or togenerate usable energy.

The inclusion of additional combustion stages in various combustionprocesses to effect the substantially complete combustion of hydrocarbonfuels has been proposed in U.S. Pat. Nos. 4,240,784 to Dauvergne;5,215,455 to Dykema and 5,271,729 to Gensler et al. However, none ofthese patents suggests improving the combustion conditions in a wetoxidation reactor or direct contact boiler with additional combustionstages to enhance the thermal efficiency of the combustion of variedmixtures of organic waste materials and low quality fuel.

U.S. Pat. No. 4,700,637 to McCartney discloses a two stage combustion orincineration process for the substantially complete combustion of lowlevel radiation waste which includes the addition of a supplementalconventional fuel and air in excess of that required for stoichiometriccombustion. Although it is disclosed that liquid waste can be processedby this system, it is essentially a dry incineration process, and thereis no suggestion that it could be used in connection with a wetoxidation process or direct contact boiler to produce usable power atenhanced thermal efficiency while producing clean, nonpolluting efflux.

The prior art, therefore, has failed to provide a combustion systememploying a wet oxidation reactor or a direct contact boiler and astoichiometric second stage burner which promotes the substantiallycomplete combustion of a range of organic materials from municipal wasteto low quality fuels at a thermal efficiency which produces usableenergy for power generation without producing undesirable environmentalpollutants. A need exists for such a combustion system.

SUMMARY OF THE INVENTION

It is a primary object of the present invention, therefore, to overcomethe disadvantages of the prior art and to provide a high thermalefficiency combustion system including a wet oxidation reactor or adirect contact boiler that produces the substantially completecombustion of low quality fuels to generate power without generatingundesirable environmental pollutants.

It is another object of the present invention to provide a combustionsystem which produces the optimum thermodynamic state of the gaseousproducts of combustion supplied for power generation.

It is a further object of the present invention to provide a combustionsystem which allows the use of different kinds of low quality fuels or amixture of different low quality fuels.

It is yet another object to provide a combustion system including astoichiometric second stage combustor to produce an optimum desireddesign temperature and pressure.

It is yet a further object of the present invention to provide a methodfor producing combustion products usable for power generation from solidwaste low quality fuels.

The aforesaid objects are achieved by providing a power generatingcombustion system and method which substantially completely combusts awide variety of different types and mixtures of fuel including a firststage combustion means for burning a selected fuel in the presence ofair and liquid at a predetermined design pressure and temperature and asecond stage stoichiometric combustion means for insuring substantiallycomplete combustion of the selected fuel to produce a desired finaltemperature of the combustion products. The first stage combustion meansis preferably a wet oxidation reactor or a direct contact boiler. Thesecond stage combustion means is a stoichiometric burner and may bepositioned in different locations and system configurations relative tothe first stage combustion means. Low quality organic fuels, includingmunicipal and industrial waste, may be burned by the combustion systemand method of the present invention to generate usable power withoutproducing undesirable environmental pollutants or toxic byproducts.

Other objects and advantages will be apparent from the followingdescription, claims and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a first embodiment of a combustionsystem according to the present invention including a first stage wetoxidation reactor;

FIG. 2 is a schematic diagram of a second embodiment of a combustionsystem according to the present invention including a first stage wetoxidation reactor;

FIG. 3 is a schematic diagram of a third embodiment of a combustionsystem according to the present invention including a first stage wetoxidation reactor; and

FIG. 4 is a schematic diagram of a fourth embodiment of a combustionsystem according to the present invention including a first stage directcontact boiler.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The combustion system of the present invention can be employed toconvert low quality fuels and organic waste products, such as thosewhich make up municipal and industrial byproducts and garbage, thatotherwise create disposal problems to gaseous products useful for theproduction of power in a power generating plant. These organic wastesand other, more conventional, fuels undergo substantially completecombustion in the combustion system of the present invention so thatclean, nonpolluting efflux is produced by the power plant and emissionsexceed EPA requirements.

The combustion system of the present invention is a highly thermallyefficient system. Therefore, it is ideally suited for use with powerplants seeking to achieve very high thermal efficiency. One type ofpower plant for which the present combustion system is especially wellsuited is the power plant based on a direct contact steam boilerdescribed in U.S. Pat. No. 4,841,721 to Patton and Shouman, the inventorherein. The disclosure of U.S. Pat. No. 4,841,721 is hereby incorporatedherein by reference.

The combustion system of the present invention employs two combustionstages to substantially completely combust the organic waste or otherfuel burned in the system. The first combustion stage in the system maybe a wet oxidation reactor. The wet oxidation process allows theoxidation of organic materials mixed with either air or oxygen in thepresence of liquid water. As the temperature of the water, which istypically a water bath in a wet oxidation reactor, is increased, thenecessary reaction residence time decreases significantly. Potentialpollutants are absorbed by the water bath in a form that can be disposedof. However, the maximum temperature that can be produced in a wetoxidation reactor is limited to the saturation temperature of the steamcorresponding to the partial pressure of the steam present in gaseousproducts of combustion. Because this maximum temperature is relativelylow, the potential thermal efficiency of a power plant using a wetoxidation process as the only combustion system would also be relativelylow.

The present invention allows the production of higher temperatures thanthose permitted by the wet oxidation process alone. A second combustionstage is added downstream of the first stage wet oxidation reactor. Thisis a stoichiometric combustion burner that uses an appropriate fuel toproduce the maximum temperature tolerated by the material from which thepower plant prime mover is constructed. The products of combustion ofthe stoichiometric second stage burner mix directly with the gaseousproducts of the wet oxidation reactor to produce the predetermineddesired temperature, which, in turn, allows the highest achievablethermal power plant efficiency to be reached.

Referring to the drawings, FIG. 1 presents a schematic diagram of oneembodiment of a combustion system 10 according to the present invention.This embodiment of the combustion system 10 includes a first stage wetoxidation reactor 12, which generally has a cylindrical configurationwith the diameter and height approximately sized for the power plantwith which the combustion system will be used. The reactor 12 includes afloat 14 which controls a valve 16 so that a constant level of liquid 18is maintained in the reactor 12. A mixture of fuel and water of theappropriate proportions for the type of fuel and the system is suppliedto the reactor 12 through line 20 at the design pressure and temperatureof the system. The float controlled valve 16 regulates fuel and waterflow into the reactor 12. Air or oxygen is supplied to the reactor 12through line 22 at the design pressure and temperature of the system.The design temperature and pressure will depend, in part, on suchfactors as the size of the reactor, the materials from which the systemis constructed and the thermal efficiency desired. Illustrative designtemperatures and pressures are described in more detail hereinbelow.

A portion of the liquid in the reactor 12 is blown off through line 24as required to maintain the quality of the mixture in the reactor.

The combustion system embodiment shown in FIG. 1 also includes a secondsection 26 downstream of the wet oxidation reactor 12. A second stagestoichiometric combustor or burner 28 is mounted within the combustionsystem second section 26. A stoichiometric mixture of a second fuel andair or oxygen is supplied to the second stage burner 28 through line 30at the system design pressure and temperature.

A third section 32, which is a mixing section, is located downstream ofthe section 26. In the third section the proper mixing of the hotproducts of combustion from the burner 28 (arrows a) with the coolergases leaving the wet oxidation reactor (arrows b) occurs. The finalgaseous products of the combustion system leave the system through line34 and are then supplied to the inlet of a power plant or directly to apower plant prime mover (not shown).

The wet oxidation reactor 12, the burner mounting section 26 and themixing section 32 are preferably constructed of two layers of materialso that the system will be able to reach the desired design temperatureand pressure. The interior layer of the reactor must be capable ofwithstanding high temperatures, while the exterior layer is not requiredto withstand high temperatures, but must be able to withstand the systemdesign pressures. Consequently, the interior layer may be formed of alow cost refractory material, such as, for example, clay. However,ceramics such as silicon nitride (Si₃ N₄) and silicon carbide (SiC) arepreferred for this purpose. The exterior of the reactor 12 and addedsections 26 and 32, may be constructed of a suitable composite materiallike steel. The ability to use these materials for the combustion systemof the present invention eliminates the need for such high temperature,expensive materials as titanium, which has made the construction ofmunicipal and industrial waste combustion systems prohibitivelyexpensive. A major advantage of the combustion system of the presentinvention is its significantly lower cost as compared to availableorganic waste and low quality fuel combustion systems.

FIGS. 2 and 3 illustrate two additional embodiments of the combustionsystem of the present invention which also include wet oxidationreactors. These combustion system embodiments are substantially the samein operation as the FIG. 1 embodiment. However, the configurations ofthe stoichiometric burner mounting and mixing sections and the locationsof the stoichiometric burners relative to the wet oxidation reactor aredifferent. These different configurations allow the present combustionsystem to be used with different power plant configurations.

FIG. 2 illustrates a second embodiment of the present combustion system.The wet oxidation reactor 42 includes a float 44 which controls a valve46 to maintain a constant level of liquid 48 in the reactor 42. Theproper water and fuel mixture is supplied to the reactor 42 through line50, and air or oxygen is supplied to the reactor 42 through line 52.Liquid is drawn off through line 54 as required to maintain the reactormixture quality. The burner mounting section 56 connects the reactor 42to the mixing section 58, which is oriented perpendicularly to thereactor 42. The second stage burner 60 is mounted where the centerlineof the mixing section 58 intersects the second burner mounting section56. Line 62 supplies a stoichiometric mixture of air or oxygen and fuelto the burner 60. Line 64 carries the gaseous mixture produced by thetwo combustion stages (not shown) to the power plant.

FIG. 3 illustrates a third embodiment 70 of the present combustionsystem. This embodiment provides a third system configuration. A wetoxidation reactor 72 includes the same float 74 and valve 76 componentsas the other embodiments. Line 78 supplies a fuel and water mixture,line 80 supplies air or oxygen and line 82 allows liquid to be drawn outof the reactor 72. The burner mounting second section 84 is in the shapeof a 180° elbow which connects the downcomer third or mixing section 86to the wet oxidation reactor 72. The second stage combustion burner 88is mounted where the centerline of mixing section 86 intersects thesecond section 84. Line 90 supplies a stoichiometric mixture of fuel andair or oxygen to the burner 88, and the mixed gaseous combustionproducts exit the system by line 92.

FIG. 4 illustrates yet a fourth embodiment of the combustion system ofthe present invention. The combustion system 100 shown diagrammaticallyin FIG. 4 employs a direct contact boiler 102 as a first combustionstage instead of a wet oxidation reactor. This first stage combustionsystem produces a mixture of wet steam and noncondensible gases. Water,air and fuel are supplied to the boiler through lines 104, 106 and 108,respectively. Moisture laden gases produced by the first stage ofcombustion are directed from the boiler 102 through line 110 to aseparator 112. In the separator 112 the moisture is separated from thegases and leaves the separator through line 114. Moisture-free gas exitsthe separator through line 116 and enters a second stage combustion area118. A stoichiometric second stage combustor or burner 120 produces thedesired design temperature at the desired design pressure.

An appropriate mixture of fuel and air or oxygen is supplied to thestoichiometric burner 120 through line 122. The flow rate of this fueland air mixture is controlled to produce the desired final temperatureof the gaseous mixture in the second stage mixing section 124. Optimummixing of the gases from the separator 112 and the combustion productsproduced by the second stage burner 120 takes place in the mixingsection 124 so that the gas exiting the second stage combustion area 118through line 126 has a substantially uniform temperature. The gasesleaving line 126 may be fed directly to the prime mover of a powergenerating plant.

The boiler 102 and the mixing section 124 are constructed of two layersof construction material. The interior layer, like the interior layer ofthe FIGS. 1-3 embodiment, is constructed of a material capable ofwithstanding the desired system temperature. The exterior layer isconstructed of a material capable of withstanding the desired systempressure.

The combustion system of the present invention effectively substantiallycompletely combusts a wide range of different types of fuels, frommunicipal garbage to industrial waste to low quality coal. It isparticularly useful for disposing of solid waste so that potentiallytoxic substances are completely burned and thus converted to nontoxicsubstances. Any reusable chemicals that are not completely combusted maybe extracted from the system and recovered. The liquid drawn off the wetoxidation reactor (line 24 in FIG. 1) could be appropriately treated torecover useful materials. The present combustion system achievessubstantially complete combustion so that only minimal ash is produced.This ash, however, is nontoxic and usable and does not require disposal.

The present combustion system achieves substantially complete combustionof the low quality fuels described herein with the combination of afirst stage wet oxidation reactor or direct contact boiler and a secondstage stoichiometric burner at system design pressures and temperatureswhich have not heretofore been employed in connection with either wetoxidation reactors or direct contact boilers. The present systemeffectively uses higher temperatures to completely combust solid wasteand the like. Table I below sets forth system design pressures andtemperatures for a typical system fuel, that is garbage or municipalsolid waste, combusted under two conditions in a wet oxidationreactor-based system. Under the first condition, pure oxygen is suppliedto the wet oxidation reactor (12 in FIG. 1) and to the second stagestoichiometric burner (28 in FIG. 1) in a stoichiometric ratio with thefuel. Under the second condition, air is supplied in stoichiometricratio to the wet oxidation reactor and to the second stage burner withthe fuel. Temperatures and pressures are measured in the mixing section(32 in FIG. 1).

                  TABLE I                                                         ______________________________________                                                   TEMPERATURE (°F.)                                                      500   550     600     650   700                                    ______________________________________                                        Oxygen Pressure (psi)                                                                       745    1150    1680  2400  3350                                 Air Pressure (psi)                                                                         1040    1600    2350  3300  4460                                 ______________________________________                                    

The combustion process of the present invention effectively combusts awide range of low quality fuels to produce gaseous combustion productsthat are usable for power generation. Although garbage and solid wasteare the fuels most likely to be used in the process described herein,low quality and high sulfur coal can be efficiently combusted by thepresent process to produce gaseous combustion products usable for powergeneration and the like. A number of coal gasification processes hasbeen proposed. However, none of the known processes is based on a wetoxidation reactor to produce gaseous combustion products from coal. Thecombustion system of the present invention can be effectively modifiedto gasify coal. In this modified process, a mixture of coal and water issupplied to a wet oxidation reactor, such as reactor 12 in FIG. 1.Controlled amounts of air or oxygen are supplied to the reactor so thatcombustion of the coal in the wet oxidation reactor is controlled. Thesecond stage stoichiometric burner (28 in FIG. 1) can be eliminated fromthis coal gasification process, and usable combustion products can beproduced at lower temperatures than those at which coal gasificationprocesses are currently conducted.

Additional modifications of the present combustion system whichsubstantially completely combust a low quality fuel or solid waste toproduce usable gaseous combustion products are contemplated to be withinthe scope of the present invention.

Industrial Applicability

The combustion system and method of the present invention will find itsprimary application in the waste disposal industry where it can be usedto substantially completely combust municipal or industrial waste toproduce combustion products useful for power generation. The presentcombustion system will also find applications in the power generatingindustry where low quality fuels and coal can be substantiallycompletely combusted to produce gaseous power-generating combustionproducts.

I claim:
 1. A highly efficient power generating combustion system whichsubstantially completely combusts a wide variety of different qualities,types and mixtures of fuels to produce combustion products having apredetermined desired uniform power-supplying temperature and apredetermined system pressure to supply usable power to the prime moverof a power generating plant, said power generating system comprising:(a)a first stage wet combustion system capable of burning a selected fuelat a selected system temperature and pressure to produce first gaseouscombustion products having a first temperature; (b) a second stagecombustion system comprising a stoichiometric burner capable of beingcontrolled to burn a second fuel and air or oxygen to produce secondgaseous combustion products having a second temperature higher than saidfirst temperature of said first gaseous combustion products, whereinsaid second stage combustion system is located downstream of andfluidically connected to said first stage combustion system; and (c) amixing section located downstream of both said first combustion systemand said second combustion system and upstream of and connected to thepower generating plant, wherein said first gaseous combustion productscontact second gaseous combustion products and are directly intermixedtherewith in said mixing section to produce final combustion productshaving said predetermined desired uniform power-supplying temperatureand said predetermined system pressure for supply to said power plantprime mover, and said power generating combustion system is configuredand sized according to the requirements of the power generating plant.2. The power generating combustion system described in claim 1, whereinthe first stage wet combustion system, the second stage combustionsystem and the mixing section are constructed to have an interior layercomprising a material capable of withstanding said predetermined desireduniform power-supplying temperature and an exterior layer comprising amaterial capable of withstanding said predetermined system pressure. 3.The power generating combustion system described in claim 2, wherein theinterior layer comprises a ceramic and the exterior layer comprisessteel.
 4. The power generating combustion system described in claim 1,wherein said first stage wet combustion system comprises a wet oxidationreactor or a direct contact boiler.
 5. The power generating combustionsystem described in claim 1, wherein said system is configured so thatsaid first stage combustion system, said second stage combustion systemand said mixing section are axially aligned along a common centrallongitudinal axis.
 6. The power generating combustion system describedin claim 1, wherein said system is configured so that said second stagecombustion system is positioned perpendicularly to said first stagecombustion system, and said mixing section is axially alignedlongitudinally with said second stage combustion system.
 7. The powergenerating combustion system described in claim 1, wherein said systemis configured so that said first stage combustion system and said mixingsection are positioned with the central axis of the first stagecombustion system parallel to the central axis of the mixing sectionwith a 180 degree elbow therebetween, and the second stage combustionsystem is located in the elbow between the first stage combustion systemand the mixing section.
 8. An efficient method for generating usablepower from a wide range of fuels and mixtures of fuels by substantiallycompletely combusting a selected fuel to produce combustion productshaving a predetermined desired uniform power-supplying temperature and apredetermined system pressure, including municipal solid waste,industrial waste and low quality fuels, including the steps of(a)supplying the selected fuel, water and a source of oxygen to a firststage wet combustion system; (b) combusting said selected fuel in saidfirst stage wet combustion system to produce first stage gaseouscombustion products having a first temperature; (c) supplying a secondfuel and a source of oxygen in stoichiometric amounts to a second stagecombustion system comprising a stoichiometric burner and combusting saidsecond fuel to produce second stage gaseous combustion products having asecond temperature that is higher than the first temperature of saidfirst stage gaseous combustion products; (d) directly contacting saidfirst stage gaseous combustion products with said second stage gaseouscombustion products and intermixing them to increase the temperature ofsaid first stage gaseous combustion products, thereby substantiallycompletely combusting said selected fuel and producing final gaseouscombustion products having said predetermined desired uniformpower-supplying temperature and said predetermined system pressure,wherein the fuel and oxygen supply to said stoichiometric burner iscontrolled to produce said second stage gaseous combustion productshaving a second temperature high enough to produce said desiredpower-supplying temperature in said final combustion products; and (e)directing said final gaseous combustion products to a prime mover of apower generating plant.